REBCO (REBa2Cu3O7-δ, RE=rare earth elements) coated conductor (CC) is one of the best candidates for building high-field magnets and
it has been improved greatly in recent years. CC overcome the grain boundary problem by using either a rolling assisted biaxially textured
substrate (RABiTS) or ion beam assisted deposition (IBAD) of a template for the REBCO. Artificial pinning centers were also introduced to
increase critical current density. Despite all these improvements, one significant residual problem is lengthwise critical current (Ic)
variations of the CCs. Characterizations of CCs can not only identify the variations, but also provide insight that can help improve the
manufacturing process. This study focuses on cross-sectional and vortex pinning variations in CCs. With the reel-to-reel Ic and magnetization
measurement system (YateStar), a systematic study has been carried out for CCs made in the last 5-6 years as this technology has rapidly
developed. We found that cross-section variations exist for almost all conductors because of width variations. But this contribution to the
total Ic variation is small. Vortex pinning variations are found to be the main reason for Ic variations, especially for conductors from
different production runs. Even for conductors from the same run, pinning variations are often present. Microscopy studies show that the
density and length of BaZrO3 (BZO) nanorods vary between different conductors even though they have nominally the same specifications.
Pinning variations in one single tape are mostly attributed to the size variations of BZO nanorods and the configurations of RE2O3
precipitates. Deconstruction of magnet coils and cables were carried out to understand the reasons for in-service degradation. The prototype
coil for the 32 T project was safely quenched more than 100 times but it degraded in 3 spontaneous quenches (conducted in an accelerated
fatigue testing campaign at ramp rates much larger than service specification). Its pancake coil deconstruction showed three extremely
localized burned regions, whose temperature went to over 800oC based on the appearance of a Cu-Ag eutectic above the damaged REBCO layer.
Transverse propagation of the damage was almost as effective as longitudinal propagation. Transmission electron microscope images show that
thicker BaZrO3 (BZO) nanorods exist near the centers of damaged zones, compared to longer and thinner BZO nanorods from normal, good regions.
Because of the lack of detailed Ic(x) characterizations of the length prior to use, the cause the cause of the coil degradation is not clear.
It is possible that local degradation of the vortex pinning initiated the final quenches but another possibility is indicated by
deconstruction of a no-insulation coil, which reached 45.5 T in a background field of 31 T. In this case no burn marks were observed but some
tapes were heavily deformed on one edge, and some joints delaminated after quenches. Transport measurements show that the deformations
correlate to Ic degradations, especially for the outer turns of pancakes. Microstructural studies reveal that the deformed (and cracked)
edges are always the one that were slit during manufacturing. It appears that small, pre-existing micro-cracks on slit edges propagate after
high-field tests. Study of individual strands of conductor on round core (CORC®) cables demonstrated their steady improvements in the last
few years. Overall cable current density, Je, has been greatly improved by replacement of 50 m by 30 m thick substrate in CCs and improved
winding procedures cause no damage to the tapes. However, some degradation may appear after cables are bent and tested in high-field (20 T).
It is found that inner layers are more vulnerable than outer layers. Winding angles and gaps strongly influence where degradations start. To
understand the failure mechanisms and establish the limiting winding conditions for CORC® cables/wires, tapes were wound on different formers
at different angles: 23o, 30o, 45o and 60o. For a 2 mm former diameter, the highest winding angle gives the least degradation while the other
three are comparable. A major defect type introduced during winding is propagation of pre-existing edge (slitting) cracks, but some
delamination under winding stress can also be seen. For the former with 2.54 mm in diameter, no propagations of pre-existing cracks or
delaminations were observed after winding. Our studies of CCs made and tested in different ways has shown that further improvement of CC and
of CORC® cables/wires can be made and also that some inherent features of the manufacture of CCs exert a strong influence on their service
performance. / A Dissertation submitted to the Program in Materials Science and Engineering in partial fulfillment of the
requirements for the degree of Doctor of Philosophy. / Fall Semester 2017. / November 17, 2017. / Includes bibliographical references. / David Larbalestier, Professor Co-Directing Dissertation; Jan Jaroszynski, Professor Co-Directing
Dissertation; Timothy Cross, University Representative; Eric Hellstrom, Committee Member; Christianne Beekman, Committee Member; Theo
Siegrist, Committee Member; Dmytro Abraimov, Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_604972 |
| Contributors | Hu, Xinbo (author), Larbalestier, D. (David) (professor co-directing dissertation), Jaroszynski, Jan (professor co-directing dissertation), Cross, Timothy A. (university representative), Hellstrom, Eric (committee member), Beekman, Christianne (committee member), Siegrist, Theo (committee member), Abraimov, Dmytro (committee member), Florida State University (degree granting institution), Graduate School (degree granting college), Program in Materials Science (degree granting departmentdgg) |
| Publisher | Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text, doctoral thesis |
| Format | 1 online resource (143 pages), computer, application/pdf |
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